1. Field of the Invention
The present invention relates to a zoom lens, and more particularly to an image pickup lens that is suitably used for an image pickup apparatus such as a video camera, a surveillance camera, a digital still camera, a broadcast camera, or a silver-salt film camera.
2. Description of the Related Art
A zoom lens having a small-size total system with a high zoom ratio is required for an image pickup optical system that is used for an image pickup apparatus such as a video camera, a surveillance camera, or a digital still camera. As a small-size zoom lens with a high zoom ratio, a four-unit zoom lens of a positive lead type that is configured by four lens units as a whole, which includes a lens unit having a positive refractive power (an inverse of a focal length) disposed closest to an object side, is known. As a four-unit zoom lens, a zoom lens which includes, in order from an object side to an image side, first to fourth lens units having positive, negative, positive, and positive refractive powers respectively is known.
A four-unit zoom lens of a rear focus type in which the second lens unit or both the second lens unit and the third lens unit in the lens units are moved to perform a magnification varying operation and also the fourth lens unit is moved to correct a variation of an image plane due to the magnification variation while focusing is known.
U.S. Pat. No. 5,712,733 and Japanese Patent Laid-Open No. 2001-116996disclose a four-unit zoom lens having a high optical performance over an entire zoom range in spite of having a small number of lenses as a whole.
It is comparatively easy for the four-unit zoom lens of the positive lead type to achieve a small size of the total system and a high zoom ratio. However, in order to obtain a high optical performance over the entire zoom range while achieving the small size of the total system and the high zoom ratio, it is important to appropriately set a refractive power of each lens unit, a lens configuration, and the like. For example, in order to obtain a high optical performance in the entire zoom range while having a small number of lenses as a whole and also reducing deterioration of the optical performance caused by a manufacturing error, it is important to appropriately set the refractive powers and the lens configurations of the third lens unit and the fourth lens unit.
If these configurations are not appropriately set, it is difficult to obtain a high optical performance over the entire zoom range while achieving a small size of the total system and a high zoom ratio. For example, in the four-unit zoom lens which has a first lens fixed during zooming, there are optical characteristics that a diameter of a light beam entering the third lens unit is increased at a wide angle end. Therefore, the third lens unit has an optical role of correcting spherical aberration or coma aberration at the wide angle end.
In this case, if the refractive power of the third lens unit is weakened and also the diameter of the light beam entering the fourth lens unit is increased, it is easy to correct the spherical aberration or the coma aberration using whole of third lens unit and the fourth lens unit at the wide angle end. However, when for example the third lens unit and the fourth lens unit are decentered due to the manufacturing error and the like, a large amount of the coma aberration caused by the decentering is generated.
On the other hand, when the refractive power of the third lens unit is strengthened, it is possible to correct the spherical aberration or the coma aberration at the wide angle end only by using the third lens unit. In this case, in order to reduce the number of the lenses of the third lens unit while appropriately performing the aberration correction, there is a method of using the third lens unit in which a positive lens and a negative lens are disposed in order from the object side to the image side and a negative refractive power is given to a lens surface closest to the image side. In this method, since a positive refractive power is ensured by a lens surface other than the lens surface closest to the image side, a curvature of the lens surface closest to the object side tends to be increased or a refractive power of an air lens between the positive lens and the negative lens tends to be increased.
As a result, a large amount of high-order spherical aberration or coma aberration tends to be generated. In this case, when manufacturing errors of parts are contained, the optical performance is significantly deteriorated. In particular, a large difference between the spherical aberrations in a short wavelength and a long wavelength is generated, and it is difficult to correct these aberrations.